1. Field of the Invention
The present invention relates to the treatment of exhausted Cr.sup.+6 oxidizing solutions by electrolysis. More particularly, the present invention relates to the electrolysis of exhausted Cr.sup.+6 solutions which contain divalent copper ions in an electrolysis cell containing carbon particle electrodes such that Cr.sup.+6 and Cu.sup.+2 and simultaneously reduced to Cr.sup.+3 and copper metal respectively.
2. Description of the Prior Art
Oxidizing solutions based upon hexavalent chromium in aqueous sulfuric acid have been employed in cleaning and etching operations in the treatment of plastics for plating, in brass finishing, in printed circuit board etching, in anodizing and in other surface treatments. As the oxidizing solutions are used, Cr.sup.+6 is reduced to Cr.sup.+3 and the dissolved solids content increases while the effective acid concentration diminishes. However, after use of such cleaning solutions, the problem which remains is disposal of the spent etching solutions, i.e. solutions whose etching rate is less than that required for a given operation.
In the past substantial quantities of chromium values have been lost through discard of the solutions. Moreover, the technology for the ultimate disposal of chromium wastes usually requires the reduction of remaining Cr.sup.+6 to Cr.sup.+3. The solution containing trivalent chromium is then treated with a base to precipitate chromium values and other extraneous metals present as the hydroxides. The solid sludge obtained has been used as a landfill material which represents a substantial waste of valuable chromium values.
The use of chromic acid-sulfuric acid etchants in the fashion described above poses significant problems for the recovery of secondary resources and for pollution control. It is significant that the entire U.S. production of primary chromium is achieved from imported ores. In fact, more than 30,000 tons of chromium are employed annually for the oxidizing treatment of various surfaces and for corrosion control measures. Because of minimal recovery technology and because of the practices of the finishing industry which utilize Cr.sup.+6 oxidizing solutions, substantial quantities of valuable secondary chromium materials are lost. Furthermore, waste chromium has long been recognized as a major pollution problem. In fact, the prevailing EPA regulations prevent the discharge of waste water containing more than 0.25 ppm Cr.sup.+6 into sewers by plants discharging more than 10,000 gal/day of aqueous chromium wastes. Still further, suitable landfill areas for chromium hydroxide sludge materials are becoming increasingly scarce, and the collection, treatment and disposal of chromium waste by contractors is expensive.
In the past one method of treating exhausted hexavalent chromium oxidizing solutions as shown by Tarjanji et al. U.S. Pat. No. 3,728,238 has involved the electrolysis of such solutions in electrolysis cells containing anode and a cathode under conditions in which the hexavalent chromium is reduced to trivalent chromium. The electrolysis is conducted in a cell which contains a bed of particles having a porosity ranging from about 40% to 80%. The particles can be electrically non-conductive, conductive or semiductive and function by facilitating the reduction process. While the reference shows the reduction of chromium values in solution it does not show a technique whereby Cr.sup.+6 and Cu.sup.+2 values in solution can be simultaneously reduced without resorting to addition of chemicals to the electrolysis bath or to separation of the anolyte from the catholyte.
Another prior art technique of reducing Cr.sup.+6 values in spent oxidizing solutions is shown by Gilby et al. U.S. Pat. No. 3,679,557. In this technique spent Cr.sup.+6 solutions are electrolyzed in a cell equipped with a carbon particle cathode and a noble metal anode thereby achieving the reduction of Cr.sup.+6 to Cr.sup.+3. While an apparently high rate of Cr.sup.+6 reduction can be achieved, the reference does not show or suggest a method by which Cr.sup.+6 and Cu.sup.+2 can both be simultaneously reduced in the same cell from solution so that copper can be electrowon and chromium can be reduced to a form suitable for disposal.
In a departure from the disclosures of the above discussed references, Bennion et al. U.S. Pat. No. 3,804,733 shows a technique of electrowinning copper from solutions also containing other impurity ions. In the technique the solution to be treated is passed through the cathode which is a carbon particle electrode while a reference electrolyte solution is passed through a carbon particle anode. The porous anode is provided with copper in ionizable form. As a direct current is impressed across the electrodes, copper in the solution to be treated is reduced to the metal while copper is oxidized to solution soluble form at the anode. Upon saturation of the cathode with reduced metal, the potential impressed across the electrode is reversed and the cell is regenerated. However, the reference provides no disclosure that an exhausted Cr.sup.+6 oxidizing solution can be treated such that Cr.sup.+6 values are reduced to Cr.sup.+3 and that divalent copper is simultaneously electrowon. Therefore, a need continues to exist for a method in which exhausted hexavalent chromium solutions containing copper ions can be electrolytically treated in order to simultaneously remove copper ion and Cr.sup.+6 values therefrom.